Several composite reverse osmosis membranes have been proposed, and such a conventional composite reverse osmosis membrane is produced by forming an activated skin layer having a substantially selective separability on a microporous support. This reverse osmosis membrane has a different structure from that of an asymmetric reverse osmosis membrane.
Several composite reverse osmosis membranes, each having been formed on a support as a thin film and comprising a polyamide obtained by an interfacial polymerization of a polyfunctional aromatic amine and polyfunctional aromatic acid halide, are disclosed in e.g., JP-A-55-147106, JP-A-62-121603, JP-A-63-218208, and JP-A-2-187135. The term "JP-A" means an "unexamined published Japanese patent application".
Other prior art references disclose composite reverse osmosis membranes wherein skin layers comprising polyamide are formed on supports, and the polyamide is obtained by an interfacial polymerization of a polyfunctional aromatic amine and a polyfunctional alicyclic acid halide (cf. JP-A-61-42308).
The composite reverse osmosis membranes described above have a high desalination property and a high water permeability, but it has further been desired for those membranes to improve the water permeability while keeping the high desalination property from the standpoint of the efficiency and lowering the cost for operation or equipment. For these requirements, various kinds of additives are proposed e.g., in JP-A-63-12310. However, in the conventional composite reverse osmosis membranes, the improvement of the water permeability is still insufficient even where these additives are used, and a composite reverse osmosis membrane having a higher water permeability has been desired.
Such a composite reverse osmosis membrane has a support on which a skin layer of polyamide obtained by interfacial polymerization between the polyfunctional aromatic amine and the aromatic or alicyclic polyfunctional halide. The polyamide of the membrane has negatively charged fixed groups. Thus, the anion exclusion rate is high while the cation is hardly excluded in a high pH region when desalination of mineral salt is conducted in a low-concentration region. In order to solve such problems, JP-A-62-266103 discloses a composite osmosis membrane which comprises an activated layer on which an organic polymer having positively charged fixed groups is coated. This membrane is manufactured to function as an adsorptive film. Thus, the predetermined efficiency will deteriorate after the membrane is used repeatedly. This membrane is especially inadequate for use as a second membrane in a two-stage reverse osmosis treatment (RO) conducted before preparing ultra-pure water for manufacturing semiconductors. In this newly-developed treatment, a negatively-charged membrane having high desalination property is used in the first stage, and the permeated liquid is supplied for the second stage. Thus, the desalination efficiency is deteriorated if the two membranes have the same characteristics.